Well, heavy traffic and not wanting to blow through red lights are two reasons. Wanting to live a little longer may be another. But that doesn't mean that you can't get in a good workout even if its less than 20mph avg. Do some decent hills and maybe power drills in a higher gear. Also fast spinning up some hills definately gets my heart rate going.
[MENTION=12814]noisebeam[/MENTION] and [MENTION=392454]CliffordK[/MENTION], are you using a Garmin for the speed chart that you posted? They're pretty cool.